1. Field of Invention
This invention relates to methods for producing sheet molding compounds, and more particularly to methods for producing sheet molding compounds utilizing a multilayered, multistructured, multipolymer release/barrier film.
2. Description of the Related Art
A significant amount of research and development efforts in the technology of reinforcing plastics and thermoplastics has been made. Encouraged by the need of the aerospace and automobile industry to produce lighter weight and more energy efficient vehicles, planes and associated apparatus, a great deal of the activity in this field has been devoted to developing plastics strong enough and with sufficient durability to replace many of the metal structural support members in such vehicles. Reinforced plastic parts must possess similar structural strength and integrity as a metal component while reducing its weight. Because of this demand, a significant research and development effort directed at developing high strength structural composites has been focused on the development of sheet molding compounds (SMC), which thereby allow rapid matched-die molding methods.
Typically SMC are comprised of an unsaturated polyester, a crosslinkable polymeric resin, chopped fiber reinforcement, and other additives of lesser importance. Such composite material is generally prepared by depositing the chopped fiber on a layer of fluid resin supported on a moving polyamide/polyolefin film. This matrix is generally sandwiched by an additional layer of film composed of a blended polyamide/polyolefin film of low crystallinity, similar to that disclosed in U.S. Pat. No. 4,444,829 issued Apr. 24, 1984. Alternatively, a common practice is to use a polyolefin film where the polyamide/polyolefin does not provide sufficient release from particular SMC compounds.
Usually such composites are passed through a series of kneading and compaction rolls and then typically rolled into large rolls. Such rolls are then wrapped in aluminum foil to prevent loss of monomers such as styrene. Following a maturation period, usually about 2-5 days during which the polyester partially cures and increases in viscosity to a suitable consistency, the SMC is then used to produce molded parts for airplanes, boats, automobiles, and the like, by cutting a piece of SMC from the roll, peeling away the carrier film, and then placing the SMC in a heated mold for molding and complete curing.
However, there are significant limitations associated with all such prior SMC producing methods and apparatuses. Due to the limited amount of polyolefin and the conventional blending of the polyolefin with a polyamide to produce a polyamide/polyolefin alloy, such as the disclosed in U.S. Pat. No. 4,444,829 and U.S. Pat. No. 4,568,505 issued Feb. 4, 1989, significant shortcomings result. The primary limitations of such prior methods is the polyamide/polyolefin does not release from many SMC compounds therefore making it necessary to use a straight polyolefin film for many applications as well as allowing significant amounts of monomers such as styrene to escape into the atmosphere. Furthermore, when a straight polyolefin film is utilized during production of SMC compounds, release is no longer a problem, however during the ensuing storage period, even more significant amounts of monomers such as styrene escape than with a polyamide/polyolefin alloy, which in either case is significant because styrene acts as a cross-linking agent for the resin and styrene is considered an environmental hazard.
Another significant limitation of prior methods is that a straight polyolefin film has far less tensile strength that a multilayered, multistructured, multipolymer release/barrier film which increases the chance of carrier film breakage. This results in more interruptions and down time in the production process and requires SMC producers to carry multiple inventories. In addition to a straight polyolefin having extremely limited resistance to styrene monomer, a film composed of a polyamide/polyolefin mix, such as that described in the aforementioned U.S. Pat. No. 4,444,829, allows far more styrene permeation than desired by SMC producers as well as the previously mentioned loss of styrene monomer through the polyolefin sections of the polyamide/polyolefin film resulting in poor product flow and unacceptable molded parts. Secondary problems associated with such prior films is the odor from the released styrene and the concomitant health and environmental hazards associated with such release. Moreover, if the ratio of polyamide to polyolefin is decreased to improve release, the resultant film allows even more styrene passage and has less tear strength, resulting in more down time and financial loss for the SMC producer.
Therefore, there is a clear need for a film capable of releasing from all SMC compounds that does not allow significant styrene monomer transmission while at the same is of high strength so as to eliminate down time and production losses associated with all prior SMC films.
Accordingly, it is the primary object of this invention to provide a method for the production of an SMC utilizing a film which is capable of releasing from all SMC compounds, does not allow significant styrene monomer transmission, and which is of very high strength. The foregoing is achieved utilizing a multilayered, multistructured, multipolymer release/barrier film either monoaxially oriented or bi-axially oriented to coat or sandwich an SMC. This allows for excellent release characteristics from all SMC compounds, eliminates down time and loss due to tearing, and can be specifically designed to meet particular SMC formula requirements thereby providing an SMC producer flexibility in controlling the styrene content of its compounds depending upon their final application, without sacrificing release characteristics or strength.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.